The invention relates to a process for preparing liquid hydrocarbons from coal.
Hydrocarbon mixtures boiling in the gasoline range can be obtained, for instance, by straight-run distillation of crude mineral oil, by conversion of heavy mineral oil fractions, for instance, by catalytic cracking, thermal cracking and hydrocracking, and by conversion of light mineral oil fractions, for instance by alkylation.
In view of the increasing need of gasoline and the decreasing reserves of mineral oil there is a great interest in processes having the potentialities of converting carbon-containing materials not based on mineral oil, such as coal, in an economically justified way into hydrocarbon mixtures boiling in the gasoline range.
It is known that carbon-containing materials, such as coal, can be converted into mixtures of carbon monoxide and hydrogen by gasification. It is further known that mixtures of carbon monoxide and hydrogen can be converted into mixtures of hydrocarbons by contacting the gas mixtures with suitable catalysts. Finally, it is known that mixtures of paraffins and olefins boiling below the gasoline range can be converted into hydrocarbon mixtures boiling in the gasoline range by contacting the mixtures first mentioned with an alkylation catalyst.
Investigation has shown that gasoline having a high octane number can be prepared from coal by combining the three above-mentioned processes, provided that the following conditions are satisfied.
First of all, the gasification of the coal should be carried out at a temperature of from 1050.degree. to 2000.degree. C. From the mixture of carbon monoxide and hydrogen thus obtained an aromatic hydrocarbon mixture should then be prepared using a catalyst which contains a crystalline aluminosilicate zeolite having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 12 and a constraint index between 1 and 12. From the aromatic hydrocarbon mixture thus obtained two fractions should then be separated, viz. an isobutane-containing gaseous fraction, which is contacted with an alkylation catalyst and an aromatic liquid fraction boiling in the gasoline range. Finally, a fraction boiling in the gasoline range is separated from the product obtained in the alkylation, and this fraction is mixed with the gasoline fraction that was separated from the reaction product of carbon monoxide and hydrogen.